For intermittent slow reproduction in which activating and braking pulses are alternately supplied to a capstan motor for tape feed of a VTR to repeat a still status and a two-field standard traveling status, if a head drum is rotated at the same speed for still and standard reproduction, lateral vibration (jitter) of an image is caused by the change in the relative speed of a video head and the tape.
To prevent such jitter of the image, as described in Japanese Utility Model Laying-Open No.4-35234, acceleration of the capstan motor is detected to increase the speed of the speed control data of the rotating drum, or deceleration of the capstan motor is detected to decrease the speed of the speed control data of the rotating drum for braking.
In this method, however, a sufficient effect cannot be ensured to reduce the jitter as time delay is caused for the actual control voltage applied to the drum motor when the capstan motor is activated, as shown in FIG. 2.
Then, common practice is to employ a method of directly applying a pulse voltage to a rotating drum control voltage together with the method described above.
FIG. 3 shows a block diagram of a conventional rotating speed control circuit for a head.
In the activation process for intermittent driving, the activating pulse is generated in a capstan intermittent driving control circuit (2) in accordance with a head switching signal generated in a head switching signal generating circuit (1) and is supplied to a capstan control PWM output generating means (3).
In capstan control PWM (Pulse Width Modulation) output generating means (3), a PWM output signal at H (at high level) is generated for a prescribed period of time, which is then smoothed by a low pass filter (LPF) and converted to a direct-current voltage signal for driving a capstan motor (4).
Thereafter, an error with respect to a target speed is calculated in accordance with an FG pulse output from capstan motor (4) in a capstan controlling means (5), a calculation result is transmitted to capstan control PWM output generating circuit (3), and the PWM output signal is output in accordance with the calculation result from capstan control PWM output generating circuit (3), so that rotation control of the capstan motor is performed.
Then, a control signal recorded on a magnetic tape is reproduced by a CTL head (6) and, a braking pulse is generated in capstan intermittent driving control circuit (2) delayed by a prescribed period of time with respect to the reproduced control signal, thereby braking capstan motor (4).
Intermittent slow reproduction is performed by a series of above described operations.
In activating capstan motor (4) (denoted by A in FIG. 4), however, a relative speed a drum motor (7) and the magnetic tape, that is, the relative speed of the rotary video head and the video signal recorded on the magnetic tape, is rapidly changed, thereby causing lateral vibration of the reproduced image.
To solve this problem, a correction pulse is generated in a drum correcting output generating circuit (8) and directly applied to a voltage which drives drum motor (7) through a diode (9) when the drum motor is activated.
In this method, a dedicated output port must be provided in a control microcomputer and a circuit component such as a diode is required, which may considerably increase a substrate mounting space and material cost.